1. Field of the Invention
The present invention relates to a method of evaluating the image quality of a synthetic aperture radar, and more specifically, to a method of accurately and quantitatively evaluating the quality of an image formed by a synthetic aperture radar.
2. Description of Related Art
In synthetic aperture radars, as is well known in the art, an antenna having a virtually large aperture is formed using an antenna having a rather small aperture. In this technique, an image radar (side-looking radar) is installed on a flight object (platform) such as an artificial satellite or an aircraft, and a radio wave is emitted by the image radar, as the flight object is moving, toward the ground in a lateral direction relative to the flight object. The image radar receives reflected radio waves as it moves, and performs synthetic aperture processing on the received radio waves in such a manner that an image equivalent to that obtained via a large-aperture antenna can be obtained. Such a synthetic aperture radar is used as an image sensor that can provide a high-resolution image under all-weather conditions.
FIG. 1 is a schematic diagram illustrating the construction of a typical synthetic aperture radar. In FIG. 1, there are shown a flight object (platform) 101 such as an artificial satellite or an aircraft, a transmitter 102 installed on this flight object, a receiver 103, a duplexer 104, a reception radio wave recorder 105 for recording radio waves received via the receiver 103, and an antenna 106.
Referring to FIG. 2, the operation principle of the synthetic aperture radar constructed with these elements will be described below. The flight object 101 such as an artificial satellite travels at a speed V along an air route or orbit L predetermined depending on a specific purpose. The small-aperture antenna 106 of the synthetic aperture radar installed on the flight object 101 emits transmission radio wave pulses at constant time intervals to at positions A.sub.0, A.sub.1, A.sub.2, . . . along the orbit L at a height h. The transmission radio wave pulse in the form of a beam with a width .beta. is emitted in a direction perpendicular to the orbit L, and it strikes, for example, an area BCED on the ground at a point A.sub.1. The transmission radio wave pulse is reflected from the ground, travels backward as a reflected wave (radar echo), and is finally received by the same antenna 106.
Reflected waves are received one after another during the flight object 101 moving at the speed V thereby observing a ground area between parallel lines l and l' a distance BC apart and thus recording, in the reception radio wave recorder 105, amplitude information as well as phase information contained in the received signal obtained at each temporal point. If a transmission pulse wave that was emitted by the flight object 101 when it was, for example, at the point A.sub.0, has arrived at a target point P to be detected, then the irradiation of the pulse wave to the target point P starts and the target point P will receive further radiation until it finally receives a pulse wave emitted by the flight object 101 at the point A.sub.2. The radio waves reflected from the target point P during this period are received by the flight object 101. The received radio waves include phase information corresponding to the relative velocity that varies continuously as well as distance information. The received signals are recorded and subjected later to batch processing (holographic processing or synthetic aperture processing) whereby the antenna can act as if it has a great aperture diameter equal to the distance between points A.sub.0 and A.sub.2 (synthetic aperture method).
As described above, signals are received successively at various points and recorded. The received signals are then synthesized such that the antenna can detect a target as if it has a great aperture size a few ten to few ten thousand times the actual aperture size of the antenna. This means that the synthetic aperture radar can have high azimuth resolution and thus can provide a clear image that would be obtained via the equivalent large-aperture antenna.
The received signal (echo signal) 201 also contains a small amount of undesirable components reflected from areas other than an area to be detected, as represented by a and b in FIG. 3. When the signal is subjected to Fourier transformation in holographic processing, folding occurs in the frequency spectrum of the signal. As a result, the above-described undesirable signal components are scattered into a processing spectrum band Fw of the received signal, as represented by a' and b' in FIG. 3. Such an undesirable echo signal reflected from a non-targeted area is referred to as azimuth ambiguity. An undesirable echo signal can also be caused by an echo signal of a transmission pulse one or more pulses earlier or one or more pulses later than a transmission pulse of interest. In this case, the undesirable echo signal is referred to as range ambiguity. If these ambiguity components become great relative to the primary signal, the resultant image becomes unclear.
From the above point of view, the image quality of a synthetic aperture radar can be evaluated using a signal S and ambiguity A, for example, by the ratio of S to A (S/A). In synthetic aperture radars, the image of an object to be detected is produced, in general, according to the steps including a step of receiving, via an antenna, a radio wave reflected from the object to be detected, and a step of performing synthetic aperture processing on the received signal data thereby producing a corresponding image. Although the data received via the antenna includes not only a signal S but also ambiguity A due to the properties of the antenna pattern and the repetition period of pulses, the ambiguity A is reduced and the signal S is enhanced by performing synthetic aperture processing on the received signal data. In principle, this processing, however, makes it is impossible to reduce the ambiguity to zero and thus there is still some residual ambiguity. It is required to evaluate the ambiguity A accurately in a quantitative fashion.
However, no one has succeeded in developing a method of quantitatively evaluating the ambiguity. Conventional techniques for evaluating the quality of an image formed by a synthetic aperture radar is based on the evaluation of the ratio S/A rather than the ambiguity itself. FIG. 4 illustrates an example of a conventional evaluation technique. A virtual image of a target represented by an open circle (.smallcircle.) in a bright area (land area for example) 301 on the left side of FIG. 4 is formed in a dark area (sea area for example) 302 adjacent to the bright area, as represented by an open circle (.smallcircle.). The virtual image is evaluated according to a visual impression of a human operator and the image quality is determined using the image values of the virtual image and the area around it.
In the above described conventional technique, however, the image quality is determined from the ratio S/A that is determined according to visual impressions of a human operator, and therefore it is impossible to quantitatively evaluate the ambiguity A. Besides, this technique can be applied only to a special image including both bright and dark areas adjacent to each other. Furthermore, although the ratio S/A depends not only on the component in a horizontal (azimuth) direction (that is, in the flight direction) but also on the component in vertical (range) direction, only the component either in the azimuth direction or in the range direction is taken into account in the above technique as shown in FIG. 3. Moreover, visual evaluation by human eyes is not reliable enough.